AMZ DS-1 mod...

[gutsofgold]

I did the AMZ "Fat" mod for the DS-1 and I liked the results. I just don't want to lower the gain THAT much. So instead of increasing R9 from 22 ohm to 1k, I went from 22 to 390 ohm. This offsets the collector voltage at the input gain stage (Q2 transistor). When ever I try to read the collector voltage on Q2 I get .789 volts which doesn't make sense. It should be somewhere in the 4V range, right? I'm assuming these transistors have the same pin out as most other NPN 2NXXXX transistors... Can you catch what I am doing wrong?

[gutsofgold]

Here's the mod I am follwing..

http://www.muzique.com/news/fat-mod-for-the-boss-ds-1/

[gutsofgold]

I guess what I am asking is... how do I measure the collector voltage of Q2 in a Boss DS-1 

[DougH]

Did you change the R6 base resistor as prescribed in the article? That will need to be changed as well, in order to get a reasonable bias voltage on the collector.

[gutsofgold]

Well that's what I am trying to figure out...what should I increase it to? In the mod he increases R9 to 1k and R6 to 150k. I didn't increase my R9 that high so I dont think R6 needs to go as high as his mod either. The problem is that I can't get a good bias voltage reading off the transistor so it's hard to try and get it to 4.5v

[DougH]

Try measuring the collector voltage with the original resistor values specified in the article, first.

[gutsofgold]

I did, and it was in the .4 v range.

[Ice-9]

You could always put a pot or trim in for R6, adjust it till you get the correct bias voltage, Then measure its resistance and replace it with a resistor with the same resistance.

[runmikeyrun]

^ a most excellent idea.  If you get a weird value with a very finicky pedal you might have to leave the trim pot.

[gutsofgold]

even with the stock resistor values I couldn't get a normal reading. It is supposed to be around 4.5v...and even when I did the AMZ mod just as he posted it, and noted that he had around 4.5v at the collector of Q2 my pedal was no where near that. but it sounded fine. so I think there is something wrong in the way I am measuring this.

I need to just read voltage from the collector to ground, correct?

[DougH]

need to just read voltage from the collector to ground, correct?

That's correct. I would suspect a wiring error, cold solder joint, solder bridge, bad or wrong-value component, etc at this point.

[5thumbs]

FWIW, once you figure out the problem causing <4.5V on the Q2 collector, you might want to check out the VR-MIJ-HG mod in my doc: http://www.diystompboxes.com/DIYFiles/up/Build_Your_Own_DS-1_Distortion.pdf.

The VR-MIJ mod was my first mod using Jack's "Fat Mod for DS-1".  In the VR-MIJ mod, I'm doubling the IC1B gain just like Keeley's "Ultra" mod.  In the VR-MIJ-HG, I double the IC1B gain yet again to get the max saturation level through the BS170 MOSFETs more to my ears' liking.

I really like Jack's "Fat Mod" because it is simple to do and adds a lot of nice character back to the treble-boosted stock DS-1 sound.  By doing the VR-MIJ + VR-MIJ-HG mods, I was able to get the gain pumped back up and get a groovy, chunky distortion that is simple and inexpensive to build...and lots of fun to play.

Good luck!